For the latest in Substrate master

基板上有removed As个有利于From/Into.An assumption is made that all types are at least 103.

根据trait SimpleArithmatic，实现了以下功能:

• From: u8, u16, u32
• TryFrom: u64, u128, usize
• TryInto: u8, u16, u32, u64, u128, usize

还提供了另一个特性以提供人体工程学

• UniqueSaturatedInto: u8, u16, u32, u64, u128
• UniqueSaturatedFrom: u64, u128

NOTE on 100 from Gav

SaturatedConversion(saturated_into和saturated_from)不应该被使用，除非你知道你在做什么，你已经思考并考虑了所有选项，并且你的用例暗示饱和是基本正确的.我唯一一次想象这种情况是在运行时算法的深处，你在逻辑上确定它不会溢出，但无法提供证据，因为它将依赖于一致的预先存在的状态.

这意味着从u32到基板特定类型的工作应该很容易:

pub fn u32_to_balance(input: u32) -> T::Balance {
    input.into()
}

对于较大的类型，您需要处理运行时的Balance类型小于可用类型的情况:

pub fn u64_to_balance_option(input: u64) -> Option<T::Balance> {
    input.try_into().ok()
}

// Note the warning above about saturated conversions
pub fn u64_to_balance_saturated(input: u64) -> T::Balance {
    input.saturated_into()
}

从T::Balance转换为rust原语时，还需要处理不兼容类型之间的转换:

pub fn balance_to_u64(input: T::Balance) -> Option<u64> {
    TryInto::<u64>::try_into(input).ok()
}

// Note the warning above about saturated conversions
pub fn balance_to_u64_saturated(input: T::Balance) -> u64 {
    input.saturated_into::<u64>()
}

For Substrate v1.0

基板提供pub trait As<T> in the sr-primitives crate:

/// Simple trait similar to `Into`, except that it can be used to convert numerics between each
/// other.
pub trait As<T> {
    /// Convert forward (ala `Into::into`).
    fn as_(self) -> T;
    /// Convert backward (ala `From::from`).
    fn sa(_: T) -> Self;
}

以下是一些如何使用它的工作示例:

impl<T: Trait> Module<T> {
    // `as_` will turn T::Balance into a u64
    pub fn balance_to_u64(input: T::Balance) -> u64 {
        input.as_()
    }

    // Being explicit, you can convert a `u64` to a T::Balance
    // using the `As` trait, with `T: u64`, and then calling `sa`
    pub fn u64_to_balance(input: u64) -> T::Balance {
        <T::Balance as As<u64>>::sa(input)
    }

    // You can also let Rust figure out what `T` is
    pub fn u64_to_balance_implied(input: u64) -> T::Balance {
        <T::Balance as As<_>>::sa(input)
    }

    // You can also let Rust figure out where `sa` is implemented
    pub fn u64_to_balance_implied_more(input: u64) -> T::Balance {
        T::Balance::sa(input)
    }
}